Metasurface‐Assisted Wireless Communication with Physical Level Information Encryption

Abstract Since the discovery of wireless telegraphy in 1897, wireless communication via electromagnetic (EM) signals has become a standard solution to address increasing demand for information transfer in modern society. With the rapid growth of EM wave manipulation technique, programmable metasurface (PM) has emerged as a new type of wireless transmitter by directly modulating digital information without complex microwave components, thus providing an alternative to simplify the conventional wireless communication system. However, the challenges of improving information security and spectrum utilization still exist. Here, a dual‐band metasurface‐assisted wireless communication scheme is introduced to provide additional physical channels for the enhancement of information security. The information is divided into several parts and transmitted through different physical channels to accomplish information encryption, greatly reducing the possibility of eavesdropping. As the proof of concept, a dual‐channel and high‐security wireless communication system based on a 1‐bit PM is established to simultaneously transmit two different parts of a picture to two receivers. Experiments show that the transmitted picture can be successfully retrieved only if the received signals of different receivers are synthetized as predefined. The proposed scheme provides a new route of employing PM in information encryption and spectrum utilization of wireless communication.


Biasing network design of the meta-atom
On the top layer, the bias neatwork is etched on the surface, as shown in Figure S1

Additional properties of the meta-atom in simulation
In order to further demonstrate that the two channels can be manipulated independently, the additional reflection amplitude and phase responses of the meta-atom are shown here. As depicted in Figure S2

More cases for receivers located in different directions
In the proposed wireless communication system, the receivers could be at any directions in the reflection space of the metasurface by applying different sets of coding matrices to drive the designed programmable metasurface (PM). Figure S3 displays examples for the simulated far-field patterns of different optimized coding matrices for receiver A (at f 1 = 2.4 GHz) and receiver B (at f 2 = 5 GHz), respectively. In Figure S3(a), the main beam of M 1-3 has a strong peak in the direction of θ 1 = 19° and transmits binary symbol '1' to receiver A, while M 1-4 produces a weak beam in the same direction and transmits binary symbol '0' to receiver A. In Figure S3(b), the main beam of M 1-5 transmits binary symbol '1' to receiver A in the direction of θ 1 = 30°, while the beam of M 1-6 transmits binary symbol '0'. As for the other physical channel of 5 GHz, the radiation beam of optimized coding matric M 2-3 has a strong peak in the direction of θ 2 = 12° and transmits binary symbol '1' to receiver B, while the beam of M 2-receiver B while the beam of M 2-4 transmits binary symbol '0'. For the proposed wireless transmission system, different values of θ 1 and θ 2 can be combined into different cases of information decryption, for example, (θ 1 , θ 2 ) = (19°, 12°) and (30°, 20°). Actually, combined with the far-field patterns depicted in Figure

Properties of the meta-atom in experiment
Experiments are also carried out to measure the electromagnetic (EM) responses of the

Transmission performance of each physical channel
In order to evaluate the transmission performance of each physical channel, two pictures are transmitted through two channels independently. Different from the informationencryption experiment, two pictures are independently translated into two control signals to drive the PM, and the received signals at receiver A (θ = -12°) and receiver B (θ = 5°) are demodulated individually to recover two transmitted pictures. It turns out that two pictures are recovered successfully at the same time with a low bit error rate of 4 6.7 10   and a maximum transmission rate of 2.083 Mbps. Moreover, the two receivers are moved around the pre-set direction to investigate the receiving range of each channel. As depicted in Figure S5(a), receiver A can successfully retrieve the transmitted picture from θ = -14° to θ = -9° (bit error rate is lower than 2 1.0 10   ), while received signals cannot be demodulated as the correct transmission information at other remote directions, demonstrating that the receiving range of channel A is about 5° around center angle θ = -12°. Similarly, the measured receiving range of channel B is also about 5° around center angle θ = 5°, as shown in Figure S5(b). As a result, 5 the receiving ranges of two channels accord well with the previously measured far-field patterns of the PM (depicted in Figure 2(h) and (i)), which further illustrate the good directivity and privacy of the transmission channels.

Experiment scenario for receivers in the same direction
In addition, to further demonstrate the reprogrammable and high-security features of the proposed wireless communication system, another experiment is carried out with receiver A and B located in the same direction. As shown in Figure S6, receiver A and receiver B are both located at θ = -12° with a slight altitude difference, while other configurations of the system are kept the same with the experimental scenario described in

Realization of dynamic wireless communication system
A flow chart is employed in Figure S7 to show the dynamic change of the wireless communication system when the azimuth angle θ is changed. Firstly, the variable azimuth angle θ is input to the pre-designed program to calculate the required coding matrixes applied onto the metasurface. Then, the corresponding control signals are generated by the FPGA based hardware controller to drive the PIN diodes on meta-atoms, and thus form the corresponding phase profile on the metasurface aperture. Finally, the reflection beams are generated by the programmable metasurface to construct transmission channels at different directions. It's worth mentioning that the hardware controller can simultaneously output hundreds signals of bias voltage. Therefore, all the PIN diodes on the metasurface can be controlled at the same time. When the receiver direction is changed, the metasurface can be adjusted to point at the receiver through the process mentioned above.